Change Files of type: to All Files by clicking
on the boxs drop down menu (See picture below.)

When you find the file that you wish to attach to the meeting,
appointment, or event; select it, by clicking on it.

Then, select Open.

The file name will be added to the attachment window.

Once the file appears in the window above, click Add Attachment
Now.Then, click Send (to mail the entry to the meeting attendees)
or Save to close the window without emailing anyone. If you
hit Save, the document will remain attached to the appointment,
meeting, or event and users will be able to view it when reading
the calendar entry.

When you are finished using Outlook, be sure to click the
Log Out Shortcut Icon located at the bottom of the
Outlook Bar on the left of the screen.